There are helical spacers (also called “slot rods (cores)” or “grooved rods (spacers)”) each carrying a plurality of optical fibers to form a cable, similar to various profiles of thermoplastic resin extruded products. The spacers of this type are constructed normally in structure wherein a tension-resistive wire such as a steel wire is located in the central part and a plurality of helical grooves extending in the longitudinal direction are formed of the thermoplastic resin and around the periphery of the tension-resistive wire. The thermoplastic resin is selected mainly from crystalline synthetic resins such as high density polyethylene and the like in terms of the physical properties of mechanical strength, weatherability, etc. and the raw material cost.
Since the helical grooves carry the optical fibers inside, the shape of the helical grooves is an extremely important point in terms of prevention of loss increase due to microbend of the optical fibers, and how accurately the shape was made was a big concern among those skilled in the art. On the other hand, since the helical grooves of the spacer are made by melting and extruding the thermoplastic resin and then cooling and solidifying the extruded resin, a cooling method is one of significant matters that affect the accuracy of the groove shape.
Conventionally employed as a cooling method of the extruded rod in the production steps of the helical spacer as described was a method using an elongated water tank provided at the two ends with spongy packings having such a clearance as not to contact the rod, in which the rod immediately after extrusion was made to pass through the water tank with cooling water overflowing from the two ends. This method, however, had the problem that there occurred incompletion of the shape or roughness of the surface, particularly, when the shape of the helical grooves formed in the outer surface of the rod was complex.
Japanese Patent Application Laid-Open No. S61-227016 describes a method in which air is introduced from one end of a pipe while aspirating the air from the other end so as to make an air stream and in which the spacer is made to pass through to be cooled and solidified. However, this method had the problem that it was difficult for this method to achieve the satisfactory cooling effect and the cooling interval became long.
Further, Japanese Patent Application Laid-Open No. H05-278096 describes a method in which air kept at a fixed temperature is introduced from one end of a constant temperature chamber to maintain the interior of the chamber at a predetermined temperature and in which a blast of constant temperature air is also delivered against the spacer passing inside the chamber to cool and solidify the spacer. This method, however, also had the problem that it was also difficult for this method to achieve the satisfactory cooling effect, the cooling interval became long, and the cost of facilities for the cooling was high.